Memory devices are typically provided as internal storage areas in computers. The term memory identifies data storage that comes in the form of integrated circuit chips. In general, memory devices contain an array of memory cells for storing data, and row and column decoder circuits coupled to the array of memory cells for accessing the array of memory cells in response to an external address.
One type of memory is a non-volatile memory known as flash memory. A flash memory is a type of EEPROM (electrically-erasable programmable read-only memory) that can be erased and reprogrammed in blocks. Many modern personal computers (PCs) have their BIOS stored on a flash memory chip so that it can easily be updated if necessary. Such a BIOS is sometimes called a flash BIOS. Flash memory is also popular in wireless electronic devices because it enables the manufacturer to support new communication protocols as they become standardized and to provide the ability to remotely upgrade the device for enhanced features.
A typical flash memory comprises a memory array that includes a large number of memory cells arranged in row and column fashion. Each of the memory cells includes a floating-gate field-effect transistor capable of holding a charge. The cells are usually grouped into blocks. Each of the cells within a block can be electrically programmed on an individual basis by charging the floating gate. The charge can be removed from the floating gate by a block erase operation. The data in a cell is determined by the presence or absence of the charge on the floating gate.
Because memory devices typically contain millions, if not billions, of memory cells, it is common to have one or more levels of multiplexing in coupling memory cells to an input/output (DQ) line of the memory device. For example, target memory cells, may be selectively coupled to a first multiplexer through their associated bit lines. Outputs of the multiplexer are often provided to sensing devices, e.g., that sense the data values of the target memory cells and provide signals indicative of the data values on outputs. However, as processes improve to increase feature densities, bit lines become smaller and closer together that can lead to couplings between bit lines and a multiplexer and couplings between the multiplexer and a sensing device to become undesirably close. For example, high voltage differences that may occur between the bit-line-to-multiplexer couplings and the multiplexer-to-sensing-device couplings, e.g., during memory array erase operations, may cause the couplings to break down.
For example, FIG. 14 illustrates a portion of a typical prior art memory device. The memory device has a memory array 1404, such as a NAND memory array. Bit lines 1444 of memory array 1404 are coupled to a multiplexer 1410 that selectively couples bit lines 1444 to a sensing device 1416. Bit lines 1444 of memory array 1404 are typically formed at a single vertical level (or metal layer) of memory array 1404.
FIG. 15 is a plan view of multiplexer 1410 coupled to sensing circuits 1561 of sensing device 1416. Bit lines 1444 are typically coupled to multiplexer 1410 at the same vertical level (or metal layer) as each other, and sensing circuits 1561 are typically coupled to multiplexer 1410 at the same vertical level (or metal layer) as bit lines 1444.
Specifically, bit line 14441 is coupled to a contact 15501 that in turn couples bit line 14441 to a first source/drain region of a multiplexer gate 14511. Bit line 14442 is coupled to a contact 15502 that in turn couples bit line 14442 to a first source/drain region of a multiplexer gate 14512. A contact 15503 couples a second source/drain region common to multiplexer gates 14511 and 14512 to a line 15101 of multiplexer 1410. Line 15101 couples contact 15503 to sense circuit 15611. Bit line 1444N-1 is coupled to a contact 1550N-2 that in turn couples bit line 1444N-1 to a first source/drain region of a multiplexer gate 1451N. Bit line 1444N is coupled to a contact 1550N-1 that in turn couples bit line 1444N to a first source/drain region of a multiplexer gate 1451N-1. A contact 1550N couples a second source/drain region common to multiplexer gates 1451N-1 and 1451N to a line 1510N of multiplexer 1410. Line 1510N couples contact 1550N to sense circuit 15612.
Note that bit lines 1444, lines 1510, and sense circuits 1561 are typically formed at a common vertical level. This can cause problems during erase operations performed on memory array 1404. For example, during an erase, a relatively high voltage, e.g., about 12 to about 20 volts is typically applied to bit lines 1444, while sensing circuits 1561, and thus lines 1510 are at about zero volts. This produces a large voltage difference between the bit lines 1444 and the lines 1510 at the circled regions 1560. When there is a small space between bit lines 1444 and the lines 1510 at the circled regions 1560, such as occurs when bit lines 1444, lines 1510, and sense circuits 1561 are at a common vertical level, breakdown problems can occur.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternative couplings between bit lines and multiplexers and between multiplexers and sensing devices.